1. Field of Invention
The present invention relates to an oxygen concentration detector and methods of using the oxygen concentration detector.
2. Description of Related Art
According to air-fuel ratio control of engines in recent years, an air-fuel ratio sensor and a catalyst are disposed in the exhaust system of the engine, and feedback control is performed such that an air-fuel ratio of exhaust gas detected by the air-fuel ratio sensor becomes equal to a target air-fuel ratio, for example, a stoichiometric air-fuel ratio, with a view to purifying the maximum possible amounts of noxious substances contained in the exhaust gas (such as HC, CO, NOx and the like) by means of the catalyst. As the air-fuel ratio sensor, a .lambda.-type air-fuel ratio sensor (referred to as the O.sub.2 sensor) is used. The .lambda.-type air-fuel ratio sensor has a Z-characteristic for determining, based on a concentration of oxygen contained in exhaust gas discharged from the engine, whether an air-fuel ratio of exhaust gas in the engine is rich or lean.
Alternatively, as the air-fuel ratio sensor, a limiting current-type oxygen concentration detecting element for outputting a limiting current proportional to the concentration of oxygen contained in exhaust gas discharged from the engine is used. The limiting current oxygen concentration detecting element detects the air-fuel ratio of exhaust gas in the engine from a concentration of oxygen, over a wide range and linearly. The limiting current oxygen concentration detecting element is effective in enhancing precision of the air-fuel ratio control and in performing control such that an air-fuel ratio of exhaust gas in the engine becomes equal to a target air-fuel ratio over a wide range of rich, stoichiometric and lean air-fuel ratios.
In order to maintain precision in detecting an air-fuel ratio, it is indispensable to keep the aforementioned air-fuel ratio sensor, namely, the O.sub.2 sensor or the limiting current type oxygen concentration detecting element, in an activated state. Normally, as soon as the engine is started, a heater attached to the O.sub.2 sensor or to the limiting current type oxygen concentration detecting element is supplied with electric power, whereby the sensor or the detecting element is heated and activated at an early stage. Thus, in order to maintain an activated state of the sensor or the detecting element, heater conduction control is performed.
According to a heater control device of an air-fuel ratio sensor disclosed in Japanese Patent Application Laid-Open No. HEI 8-278279, during an initial conduction period for a heater, all the electric power, that is, electric power with a duty ratio of 100%, is supplied to the heater for early activation of an element of the air-fuel ratio sensor, until the heater reaches a predetermined temperature. If the heater reaches the predetermined temperature, electric power corresponding to the temperature of the heater is supplied to the heater. If the sensor element reaches a predetermined temperature, electric power corresponding to the temperature of the element of the air-fuel ratio sensor is supplied to the heater.
However, in the heater control device of the air-fuel ratio sensor disclosed in Japanese Patent Application Laid-Open No. HEI 8-278279 mentioned above, at the time of the cold starting operation of the engine, water that has condensed in a catalyst provided upstream in an exhaust pipe either accumulates in a bottom portion of the exhaust pipe, or sticks to a wall surface of the exhaust pipe. If the exhaust system has not been warmed up at the time of the cold starting operation of the engine, the condensed water splashes together with exhaust gas and passes through small holes in a protector cover attached to the exhaust pipe so as to surround the air-fuel ratio sensor that is disposed downstream of the catalyst in the exhaust pipe. Thus, the sensor element in the protector cover is wetted and quenched, so that a difference in temperature between the heater and the air-fuel ratio sensor increases abruptly. As a result, the element of the air-fuel ratio sensor may crack due to thermal shock. Accordingly, in order to prevent the element from cracking due to thermal shock, it is important to detect a wet condition of the sensor element.